The present invention is directed toward endless fabrics, and more particularly, fabrics used as industrial process fabrics in the production of, among other things, wet laid products such as paper, paper board, and sanitary tissue and towel products; in the production of wet laid and dry laid pulp; in processes related to papermaking such as those using sludge filters and chemiwashers; in the production of tissue and towel products made by through-air drying processes; and in the production of nonwovens produced by hydroentangling (wet process), meltblowing, spunbonding, and air laid needle punching. Such industrial process fabrics include, but are not limited to nonwoven felts; embossing, conveying, and support fabrics used in processes for producing nonwovens; filtration fabrics and filtration cloths. The term xe2x80x9cindustrial process fabricsxe2x80x9d also includes but is not limited to all other paper machine fabrics (forming, pressing and dryer fabrics) for transporting the pulp slurry through all stages of the papermaking process. Specifically, the present invention is related to fabrics of the variety that improve fluid management by having voids on the backside thereof and/or internal void patterns embossed onto the fabric.
During the papermaking process, a cellulosic fibrous web is formed by depositing a fibrous slurry, that is, an aqueous dispersion of cellulose fibers, onto a moving forming fabric in the forming section of a paper machine. A large amount of water is drained from the slurry through the forming fabric, leaving the cellulosic fibrous web on the surface of the forming fabric. Typically, the newly formed cellulosic fibrous web proceeds from the forming section to a press section, which includes a series of press nips. The cellulosic fibrous web passes through the press nips supported by a press fabric, or, as is often the case, between two press fabrics. In the press nips, the cellulosic fibrous web is subjected to compressive forces which squeeze water therefrom, and which adhere the cellulosic fibers in the web to one another to turn the cellulosic fibrous web into a paper sheet. The water is accepted by the press fabric or fabrics and, ideally, does not return to the paper sheet.
In some applications, the conventional press nip has been replaced by long nip presses (LNP""s) The LNP consists of a roll, the belt, and a pressure shoe, which faces toward the roll and applies pressure to the fibrous webs and web-transporting papermaker""s press fabric or fabrics in the nip. Due to their dimensions, LNP""s offer a greater pressing area than what is available with a conventional press nip formed by two press rolls. The belts that run on LNP""s are known as shoe press belts. The belts are coated on at least one side with a resin rendering the belt impermeable to oil, water and air, and they may be coated on both sides. Examples of these kinds of belts are known in the art. U.S. Pat. Nos. 5,234,551 and 5,238,537 disclose shoe press belts on an LNP.
The paper sheet finally proceeds to a dryer section, which may include at least one series of rotatable dryer drums or cylinders, which are internally heated by steam. The newly formed paper sheet is directed in a serpentine path sequentially around each of the drums by a dryer fabric, which holds the paper sheet closely against the surfaces of the drums. The heated drums reduce the water content of the paper sheet to a desirable level through evaporation.
It should be appreciated that forming, pressing, and dryer fabrics all take the form of endless loops on the paper machine and function in the manner of conveyors. It should further be appreciated that paper manufacture is a continuous process which proceeds at considerable speed. That is to say, the fibrous slurry is continuously deposited onto the forming fabric in the forming section, while a newly manufactured paper sheet is continuously wound onto rolls after it exits from the dryer section.
In the production of some paper products, such as paper towels, facial tissues and paper napkins, through-air-drying for example augments or replaces the press dewatering described above. In through-air drying, the newly formed cellulosic fibrous web is transferred from the forming fabric directly to an air-pervious through-air-drying (TAD) fabric. Heated air is directed through the cellulosic fibrous web and through the TAD fabric to continue the dewatering process. The air molds the towels or tissues to the topography of the TAD fabric, giving the web a three-dimensional structure.
In other applications, the fabric may be used in the production of wetlaid, drylaid, melt blown and spunbonded nonwoven textiles.
Depending upon the product being produced, it may be desirable to have a pattern thereon. Passing the product through a two roll nip having at least one roll having a pattern thereon which is imprinting onto the product or paper is well known. Examples of this method is shown in U.S. Pat. Nos. 4,526,652; 5,126,015; and 5,766,416
This may also, however, be accomplished through the use of embossed fabrics which serve to imprint the embossment onto the product being produced. For example, an early TAD fabric as described in U.S. Pat. No. 3,301,746 created a multi-region structure in the web by imprinting the knuckle pattern of its weave thereon.
An improvement on this was the inclusion of a resinous frame work on the woven substrate of the fabric. Examples of this type fabric are shown in U.S. Pat. Nos. 4,514,345; 4,528,239; 4,529,480; 4,637,859; and 5,066,532.
Another method of providing an embossment on a fabric is shown in WO 98/27277 which discloses a papermaker""s fabric comprising a batt of fibers with the fabric having an embossed surface. The batt of fibers are heated with a pattern imprinted thereon while in a molten state. An improvement on this can be found in WO 99/09247.
Alternatively, the fabric may be a laminated structure with the top layer being embossed as disclosed in U.S. Pat. No. 4,541,895.
The present invention is an industrial process fabric designed for use as a forming, pressing, drying, TAD, pulp forming, or an engineered fabric used in the production of nonwoven textiles, which is in the form of an endless loop and functions in the manner of a conveyor. The fabric of the invention may also be used in sludge dewatering or in a Double Nip Thickener (xe2x80x9cDNTxe2x80x9d), which dewaters de-inked paper pulp. The fabric may be itself embossed with pre-selected topographic features in a pattern suited for the end product and its intended use.
In one aspect of the invention, the industrial process fabric has an embossed backside and is used in combination with a vented or non-vented shoe press belt. When the belt has a smooth or blind drilled surface, the press fabric embossments on the backside is advantageous to increase water removal. The pattern of the embossments on the backside may vary as will be discussed.
In another aspect of the invention, two initially distinct, independent fabrics are joined together by known processes, such as needling. Each of the fabrics has an embossed pattern on one of its surfaces. The fabrics are laminated together such that the embossed patterns are in contact with each other, creating a pattern of voids within the laminated fabric, which the skilled artisan can arrange as necessary to manipulate the properties of the fabric. For example, the patterns of the fabrics could be matching and complementary, with the embossed pattern of one fabric lining up with the embossed pattern of the second fabric. The voids or valleys of each fabric would therefore be in alignment with each other. The internal voids thus formed within the fabric laminate would create water receptacles within the fabric. This matching, complementary alignment is just one of an infinite number of possibilities.
In another embodiment, the patterns of two fabrics may be matching and offset from each other, at a desired angle. For example, a 90xc2x0 orientation would promote steady state pressing properties. The two opposing embossed patterns would create a xe2x80x9cbridgexe2x80x9d effect inside the fabric, preventing the two fabrics that form the laminate from nesting into each other. This results in better caliper retention, improved water handling, longer fabric life, and an easier-to-clean fabric.
In another embodiment, the patterns need not be matching, and could be aligned in a pre-selected pattern or randomly. An infinite number of arrangements are possible, since embossing technology permits the formation of virtually any possible pattern, which can then be joined with any other possible pattern.
Embossed fabrics may be prepared through the use of a device having embossments thereon which are heated having two opposed elements between which the fabric may be compressed at pre-selected levels of compression for pre-selected time intervals. Alternatively, the fabric can be pre-heated before being embossed. For example, embossment may be provided by a two-roll calendar, one or both rolls of which may be engraved or etched, which allows for continuous embossing. In addition, the fabric may include a low melt fiber, a fusible adhesive web or spray adhesive which can be used to reinforce and maintain the embossed pattern in the fabric while the fabric is functioning in its intended use.
Alternatively, a platen press, with upper and lower platens might also be used if the application warrants it. An embossing medium is used which has a pre-selected embossing pattern, and is capable of being readily changed from one embossing pattern to another, for example, by changing the engraved calendar rolls. In addition, the embossing method provides versatility in making desired embossed fabrics for multiple applications. The properties of the desired embossed fabric depend upon the control of certain process variables under which embossing takes place and selection of the substrate. The process variables include time, temperature, pressure, gap setting and roll composition.